NREL-Led Research Effort Creates New Alloys, Phase Diagram

June 8, 2017

A multi-institutional team led by NREL discovered a way to create new alloys that
could form the basis of next-generation semiconductors. The NREL team includes (left
to right) Stephan Lany, Aaron Holder, Paul Ndione, and Andriy Zakutayev.

A multi-institutional team led by the U.S. Department of Energy’s National Renewable
Energy Laboratory (NREL) discovered a way to create new alloys that could form the
basis of next-generation semiconductors.

Semiconductor alloys already exist—often made from a combination of materials with
similar atomic arrangements—but until now researchers believed it was unrealistic
to make alloys of certain constituents.

Scientists connected to the Center for Next Generation of Materials by Design (CNGMD)
made the breakthrough and took the idea from theory to reality. An Energy Frontier
Research Center, which is supported by the Energy Department’s Office of Science and
researchers from NREL, the Colorado School of Mines, Harvard University, Lawrence
Berkeley National Laboratory, Massachusetts Institute of Technology, Oregon State
University, and SLAC National Accelerator Laboratory.

“It’s a really nice example of what happens when you bring different institutions
with different capabilities together,” said Holder, who also is affiliated with the
University of Colorado, Boulder. His co-authors from NREL are Stephan Lany, Sebastian
Siol, Paul Ndione, Haowei Peng, William Tumas, John Perkins, David Ginley, and Andriy
Zakutayev.

A mismatch between atomic arrangements previously thwarted the creation of certain
alloys. Researchers with CNGMD were able to create an alloy of manganese oxide (MnO)
and zinc oxide (ZnO), even though their atomic structures are very different. The
new alloy will absorb a significant fraction of natural sunlight, although separately
neither MnO nor ZnO can. “It’s a very rewarding kind of research when you work as
a team, predict a material computationally, and make it happen in the lab,” Lany said.

Using heat, blending a small percent of MnO with ZnO already is possible, but reaching
a 1:1 mix would require temperatures far greater than 1,000 degrees Celsius (1,832
degrees Fahrenheit), and the materials would separate again as they cool.

The scientists, who also created an alloy of tin sulfide and calcium sulfide, deposited
these alloys as thin films using pulsed laser deposition and magnetron sputtering.
Neither method required such high temperatures. “We show that commercial thin film
deposition methods can be used to fabricate heterostructural alloys, opening a path
to their use in real-world semiconductor applications,” co-author Zakutayev said.

The research yielded a first look at the phase diagram for heterostructural alloys,
revealing a predictive route for properties of other alloys along with a large area
of metastability that keeps the elements combined. “The alloy persists across this
entire space even though thermodynamically it should phase separate and decompose,”
Holder said.

Funding for the research came from the U.S. Department of Energy’s Office of Science.

NREL is the U.S. Department of Energy's primary national laboratory for renewable
energy and energy efficiency research and development. NREL is operated for the Energy
Department by The Alliance for Sustainable Energy, LLC.